The importance to study and characterize semiconductors (bulk or thin film), semiconductor heterostructures (superlattices, quantum wells, heterojunctions) and semiconductor interfaces (Schottky barriers, metal-insulator-semiconductors, semiconductor-electrolyte, semiconductor-vacuum, etc.) assumes ever-greater significance, particularly as many of these semiconductors and semiconductor microstructures are fabricated by modern thin-film techniques such as molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), etc.
The materials and interfaces grown by MBE and MOCVD as well as other methods can be characterized by a variety of optical, electronic and structural methods including photoluminescence, photoluminescence excitation spectroscopy, absorption spectroscopy, modulation spectroscopy, Raman and resonant Raman scattering, cyclotron resonance, Hall effect, transmission electron microscopy, etc. Each of these tools provides specific information about the material of interest. For characterization purposes the experimental tools should be as simple and informative as possible. Many of the methods mentioned above are specialized and sometimes difficult to employ. Because of its simplicity and proven utility, photoreflectance has recently gained importance for the evaluation of semiconductor thin films and heterostructures.
In modulation spectroscopy in which the derivative with respect to some parameters is evaluated, uninteresting background structure is eliminated in favor of sharp lines corresponding to specific transitions between energy levels in the semiconductors and semiconductor microstructures. Also, weak features that may not have been seen in the absolute spectra are enhanced. While it is difficult to calculate a full reflectance (or transmittance) spectrum, it is possible to account for the lineshape of localized spectral features of modulation spectroscopy. The ability to fit the lineshape is an important advantage of modulation spectroscopy. Lineshape fits yield accurate values of the semiconductor energy gap as well as of the broadening parameter. In addition, since "external" modulation spectroscopy is the a.c. response of the system to the modulating parameter, photoreflectance also provides information in the other modulation variables such as phase, modulation frequency, modulation amplitude, modulation wavelength.
In photoreflectance, the built-in electric field of the materials is modulated by the photo-injection of electron-hole pairs created by a pump beam of wavelength .lambda..sub.p which is chopped at frequency .OMEGA..sub.m. Experiments have indicated that photoreflectance is due to the modulation of the built-in electric field through a recombination of the minority species with charge in traps. Thus, by measuring the dependence of the photoreflectance signal on .OMEGA..sub.m it is possible to gain information about trap times with the use of photoreflectance.
Photoreflectance, a contactless form of electromodulation, has been found to be a powerful tool to study the interface electric field distribution in semiconductor structures. In photoreflectance, the electric field in the material is modulated by the photo-injection of electron-hole pairs by a pump beam chopped at frequency .OMEGA..sub.m. It has already been demonstrated that photoreflectance is a form of electroreflectance yielding sharp, derivative-like spectra in the region of interband transitions in bulk or thin film semiconductors. Since photoreflectance is the a.c. response of the system to the modulating electric field, there is also important information in the other modulation parameters such as modulation frequency (.OMEGA..sub.m), pump beam wavelength (.lambda..sub.p), pump beam intensity (I.sub.p), etc.
In a photoreflectance apparatus as disclosed in our copending application Ser. No. 07/382,191, filed Jul. 20, 1989 and entitled "Method and Apparatus for Determining a Material's Characteristics by Photoreflectance," the subject matter of which is incorporated herein by reference, a mechanical chopper was used to modulate the pump beam. However, the modulation frequency .OMEGA..sub.m was limited to about 4,000 Hz. It has now been discovered that by varying both pump beam wavelength .lambda..sub.p and the modulation frequency .OMEGA..sub.m up to a value of 100 KHz by the use of an acousto-optical modulator, it is possible to identify the component layers, their quality and the properties of the various interfaces.
The present invention is therefore concerned with further improving the prior art apparatus to achieve improved results on the materials' determination and to gain additional information on the characteristics of the materials examined with greater accuracy and reliability.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for determining the characteristics of certain materials by photoreflectance which avoid by simple means the shortcomings encountered with the prior art apparatus and methods in the use thereof.
Another object of the present invention resides in an improved apparatus which permits determination of the characteristics of semi-insulating substrates and of undoped buffer layers and of the interface states therebetween.
A further object of the present invention resides in an apparatus utilizing photoreflectance for characterizing conductivity or semi-insulating structures as found, for example, in enhancement mode MESFET and HEMT devices.
Still another object of the present invention resides in an apparatus which can be used for in-situ characterization of epigrown surfaces and interfaces.
Another object of the present invention resides in a method for determining characteristics of certain materials, such as semiconductor materials and semiconductor heterostructures, which is simple in use, reliable in its operation and accurate in the results obtained therewith.
Still another object of the present invention resides in a method based on photoreflectance which permits continuous in-situ monitoring of the manufacture of materials, such as semiconductor materials, that eliminates the shortcomings and drawbacks encountered with the prior art systems.
A further object of the present invention resides in a method based on photoreflectance which permits accurate quality control in the manufacture of semiconductor materials.